Carbon nanotubes which are molecular-scale tubes of graphitic carbon have over the past decade attracted much academic and industrial attention due to their unique structure. CNT's (carbon nanotubes) due to intrinsic mechanical and transport properties find various applications viz. as electrode materials, composite materials, nanoelectronics, nanosensors etc. Nanowires that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length are developed as complementary to CNT's. They are good conductors or semi-conductors and are used in electronics. Considerable effort has been made to explore various heterostructures with the combination of CNTs and NWs. One dimensional (1D) nano-heterostructures have been the focus of research in recent years in the field of nano-electrotechnology, mainly due to their improved properties than the individual ones when considered meticulously separate. Owing to the fascinating properties and applications in a variety of techno-commercially important fields like nanoelectronics, Li-ion batteries, solar cells etc. carbon nanotubes (CNTs) and nanostructured silicon are seen as the most suitable candidates whose properties can be coupled to have a widespread impact. Since both silicon and carbon belong to the same periodic table and moreover their material properties and processing methods are known, for the integration of CNTs with silicon nanotechnology, high aspect ratio silicon nanowires (SiNWs) are preferred as it can offer direct blending of 1D electron transport inherently present in both the systems.
Article titled “Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires” by Jiangtao Hu, Min Ouyang et. al in Nature, 1999, 399, 48 describes Fe-catalyzed fabrication of silicon nanowires with single or multiwalledNTs (nano tubes). In the process NTs were grown from the ends of the SiNWs using ethylene at 600° C.
Article titled “Synthesis of single-walled carbon nanotube networks on silicon nanowires” by H. Yoshida, T. Uchiyama, et. al in Solid State Comm., 2007, 141,632 relates to synthesis of single-walled carbon nanotubes (SWNTs) on silicon nanowires (SiNWs) by ethanol chemical vapor deposition (CVD) using Co catalysts nanoparticles.
Article titled “Fabrication of carbon nanotube/silicon nanowire array heterojunctions and their silicon nanowire length dependent photoresponses” by Yang Cao, Jun-Hui He et. al Chemical Physics Letters Volume 501, Issues 4-6, 7 January 2011, Pages 461-465 relates to fabricated heterojunction structures, which consists of a double-walled carbon nanotube (DWCNT) thin film coated either on an n-type silicon wafer or an n-type silicon nanowires (SiNW) array with varied lengths.
Article titled “Field emission properties of carbon nanotubes grown on silicon nanowire arrays” by Yuming Liu and Shoushan Fan in Solid State Communications, Volume 133, Issue 2, January 2005, Pages 131-134 discloses synthesis of carbon nanotubes on the silicon nanowire arrays, which are fabricated on silicon substrate, by chemical vapor depositing SiCl4 and H2 gases in the presence of Au catalysts. The tangled carbon nanotubes are grown directly from the surface of Si nanowires.
An article titled “Chemical Vapor Deposition of Individual Single-Walled Carbon Nanotubes Using Nickel Sulfate as Catalyst Precursor” by L. W. Liu, J. H. Fang, et. al in J. Phys. Chem. B 2004, 108, 18460-18462 discloses synthesis of individual single-walled carbon nanotubes (SWNTs) directly on a Si/SiO2 substrate by chemical vapor deposition using methane as the feedstock and nickel sulfate as the catalyst precursor.
Article titled “On the Growth and Microstructure of Carbon Nanotubes Grown by Thermal Chemical Vapor Deposition” by Sangeeta Handuja, P. Srivastava in Nanoscale Res Lett. 2010 July; 5(7): 1211-1216. Published online 2010 May 15. describes deposition of Carbon nanotubes (CNTs) on various substrates, namely untreated silicon and quartz, Fe-deposited silicon and quartz, HF-treated silicon, silicon nitride-deposited silicon, copper foil, and stainless steel mesh using thermal chemical vapor deposition technique.
The said prior arts have the following disadvantages: (i) In nano heterostructures of SiNW-CNT having multiple contacts, continuous electron transport from one end of nanowire to the other end of nanotube through heterojunction is lost and the electron path is shared by more than one nanotube, nanowire or both. (ii) In nano hetero structures of SiNW-CNT comprising of a single SiNW and a single CNT, they however have used Fe-based catalyst for the growth of nanotubes. In such cases it is observed that there will not be any direct junction between SiNWs and CNT and the electron has to cross two barriers i.e. one between SiNWs and Fe and second between Fe and CNT before reaching the other end.
Hence, there is need of a process where a direct contact between SiNW and CNT with the heterojunction area in nanometer scale is advantageous for the development of high throughput single electronic devices with enhanced electron field emission characteristics.